The liquid crystal display (LCD) technology is always applied to the notebook, the mobile phone, the digital camera, the video camera, the PDA, and various portable electronic products. The liquid crystal display panel (LCD panel) is the most essential display part for this domain. One feature of the LCD panel is its need of a back lighting-source. In the large-sized LCD products (above 20 inch), a bottom lighting module is usually used as the back lighting-source of the LCD.
Due to rapid advance in the electronic manufacture technology, a breakthrough in the light emitting diodes (LEDs) helps to improve the display ability of the LCD screens. Since the LED can emit visible light, such as red, blue and green, it can be foreseen in the near future that the LED will replace the high-cost color filters presently used in the LCD screen. In view of lesser power consumption but providing higher lighting efficiency as well as longer service life, the roles of the LED in the LCD production become significantly.
Referring to FIG. 1, a partially fragmentary view of a conventional bottom lighting module 10 is shown to include a back bezel 12, a printed circuit board 13, at least one light emitting diode (LED) 14, a reflective plate 16 and a diffusion sheet 18. As shown, the printed circuit board 13 is electrically connected to the LED 14.
The printed circuit board 13 set on the back bezel 12 has a plurality of holes 131 for mounting the LEDs 14. The LED 14 has an upper light-emitting portion 141 and a lower LED base 142. The light-emitting portion 141 is located above the printed circuit board 13. The LED base 142 is a main heat source of the LED 14, and is located under the printed circuit board 13 so as to avoid its heat to influence the optical efficiency of LED module 10.
The reflective plate 16 adhered to the printed circuit board 13 has a plurality of cavities 161 respective to expose the holes 131 of the printed circuit board 13 as well as the light-emitting portions 141 of the LEDs 14, in which each of the light-emitting portion 141 is protruding to the reflective plate 16. Through the reflective plate 16, the lights of LED 14 can reflect upward and thus make the light to be fully utilized.
The light diffusing plate 18 is mounted on the back bezel 12. By utilizing the difference in reflectivity between the diffusing plate 18 and the air, part of the light hitting the diffusing plate 18 can be reflect reciprocally between the diffusing plate 18 and the reflective plate 16 so as to thoroughly mix the light emitted by the LED 14. Upon such an arrangement, the light leaving the diffusing plate 18 can have homogeneous brightness and color-ness.
FIG. 2 illustrates a partially fragmentary view of a double-layer diffusing plate of a conventional lighting module, in which the LED 14 emits light beams in a sidewise manner and thus majority of the light beams will not be reflected upward. Unlike the previous conventional lighting module 20, this conventional lighting module 20 shown in FIG. 2 has an auxiliary diffusing plate 22 disposed between the reflective plate 16 and the diffusing plate 18, but located much closer to the former such that the auxiliary diffusing plate 22 can provide a wider angle of light reflection. The wider angle of reflection enhances scattering of the light beams so that almost the entire light beams can be used to provide effective brightness. Due to the aforesaid reasons, the conventional LED lighting module of FIG. 2 can provide a better color mixing effect. It is why the LED module of FIG. 2 is preferably selected to display the LCD system.
Presently, the LED module is used only in the small-sized LCD device, such as a handheld PDA with an LCD screen. However, researchers in the LCD manufacturing field are harsh to solve possible difficulties in the LED modules that will be widely employed in the large-sized LCD device some day.
As illustrated in FIGS. 1 and 2, increase in the dimension of the bottom lighting module consequently requires a larger number of LEDs. In other words, sizes of the reflective plate 16, the diffusing plate 18, and the auxiliary diffusing plate 22 must also be increased proportionally. As a consequence of the dimension increase, the middle portion of any aforesaid plate would be vulnerable to bend downward by virtue of its own gravity. The housing has to burden a relatively large amount of weight and subsequently its rigidity would be reduced. In addition, the downward bending of these plates can be aggravated since the reflective plate 16, and the diffusing plates 18, 22 are all made of glass or acrylic materials which are susceptible to bend. The downward bending of these plates within the backlight module can finally result in uneven scattering of light beams in the backlight module, and hence degrading in the display ability of the LCD device would be inevitable.